Variability in estuarine vertical mixing and its impact on suspended sediment flux in weakly stratified estuaries

Xiaoyan Wei1, Megan Williams1,2, Henk M. Schuttelaars3, Jennifer M. Brown1, Peter D Thorne1 and Laurent Amoudry1, (1)National Oceanography Centre, Liverpool, United Kingdom, (2)Universidad Técnica Federico Santa María, Valparaíso, Chile, (3)Delft Institute of Applied Mathematics, Delft, Netherlands
Abstract:
Vertical mixing of momentum and density is closely linked to the interplay between turbulence and stratification. This mixing can significantly influence the tidal flow, estuarine circulation, salt intrusion, and sediment transport. Hence, advancing understanding of the interactions between the vertical mixing, turbulence and stratification is essential for predicting the response of estuarine sediment transport to natural changes (e.g., storms, spring-neap tidal variations, fluctuating river discharge, sea level rise) and human activities (e.g., land reclamation, damming).

The present study will elucidate how the spatially and temporally varying system of vertical mixing, tidal/residual flow and salinity influences the suspended sediment flux in weakly stratified estuaries. This is achieved by extending the semi-analytical model of Wei et al. (2018). We include impacts of stratification and flow velocities on the vertical mixing via linking eddy viscosity/diffusivity to the Richardson number. We also take into account the residual circulation due to covariance between the eddy viscosity and shear. The semi-analytical model allows for an analytic decomposition of processes governing estuarine circulations and sediment transport, therefore providing a fundamental investigation into the influence of varying vertical mixing on residual flow, salinity, and sediment transport.

The model is calibrated with the field measurements (pressure, velocity, turbulence, salinity and suspended sediment concentration) taken in the Blackwater estuary, which is a relatively small sized, meso-tidal estuary in Essex, the UK. A comparison to a larger sized, micro-tidal estuary with characteristics similar to the Delaware estuary will be presented.

Reference

Wei, X., Kumar, M. and Schuttelaars, H.M., Three‐dimensional sediment dynamics in well‐mixed estuaries: importance of the internally generated overtide, spatial settling lag, and gravitational circulation. Journal of Geophysical Research: Oceans, 2018, 123 (2), 1062-1090.